114 PRACTICAL ZOOLOGY 



hardens in the air, forming a thread. The silk glands are situated 

 in the abdomen and cause the large size of this part of the body. 



How Insects are Captured. The webs of the cobweb spiders 

 catch many small animals, mostly insects, but the spider itself 

 never seems to become entangled in its own web. This is prob- 

 ably because of the peculiar structure of the foot (Fig. 61, B). 

 The hunting spiders live principally on insects. The struggles 

 of the captured animals are soon stopped by a poisonous secre- 

 tion which is injected into them. This poison is formed in 

 poison glands situated in the head (Fig. 60, 20), and forced out 

 through the first pair of appendages, the chelicerae (Fig. 60, 19). 

 When the captured insect has become quiet, the spiders suck 

 out the juices into the alimentary canal by means of a sucking 

 stomach (Fig. 60, 2). 



Sense Organs. Spiders are, of course, aided by sense organs 

 in obtaining their food. Hairs that are sensitive to touch are 

 generally distributed over the body. The eyes, however, are the 

 principal organs of sense. There are usually eight (Fig. 61, C), 

 and they differ in size and arrangement in different species. 

 Spiders apparently can see objects distinctly only at a distance 

 of four or five inches. 



Respiration. Since spiders are terrestrial animals, they 

 must be able to breathe in the air. For this purpose they are 

 supplied with tracheae similar to those of insects (see p. 15), 

 but in addition they possess book lungs which are present only 

 in spiders. The book lungs (Fig. 60, 12), of which there are usu- 

 ally two, are sacs, each containing generally from fifteen to 

 twenty leaf-like horizontal shelves through which the blood 

 circulates. Air, entering through the external openings, is thus 

 brought into close relationship with the blood. 



Reproduction. The eggs of spiders are inclosed in a silk 

 cocoon which varies much in shape and color in different species. 

 Some spiders hang it in the web, others attach it to plants or 

 stones, and others carry it with them, either in the mandibles 

 or attached to the spinnerets. The young remain in the cocoon 



